Current limiting fuse with fuse element with a diamond shaped cutout

ABSTRACT

An electrical current limiting fuse of the type having fuse elements disposed in a pulverulent arc quenching material for melting after a predetermined level of current has been attained and sustained for a predetermined period of time. A fuse element of the current limiting fuse is in a form of an elongated ribbon of fuse material having a diamond shaped central opening. The diamond shaped central opening provides a pair of parallel current paths near either side of the fuse element. The diamond shaped element has two oppositely disposed angles of approximately 37 1/2 degrees approximately either side edge of the fuse material.

United States Patent 1191 Blewitt et al.

[451 Nov. 19, 1974 CURRENT LIMITING FUSE WITH FUSE ELEMENT WITH A DIAMOND SHAPED CUTOUT Inventors: Donald D. Blewitt, Pittsburgh;

Woodrow G. Shaw, Export, both of Pa.

Westinghouse Electric Corporation, Pittsburgh, Pa.

Filed: Sept. 28, 1973 Appl. No.: 401,765

Assignee:

US Cl 337/295, 337/l59, 337/290 Int. Cl. H0lh 85/04 Field of. Search 337/159, 160, 161, 290,

, References Cited UNITED STATES PATENTS 8/1918 Cole 337/295 2/1956 Kozacka. 337/295 X 12/1956 Sundt 337/295 X FOREIGN PATENTS OR APPLICATIONS 1,193,154 5/1965 Germany 337/295 Primary Examiner-J. D. Miller Assistant Examiner-Fred E. Bell Attorney, Agent, or Firm-M. J. Moran [5 7 ABSTRACT An electrical current limiting fuse of the type having fuse elements disposed in a pulverulent arc quenching material for melting after a predetermined level of current has been attained and sustained for a predetermined period of time. A fuse element of the current limiting fuse is in a form of an elongated ribbon of fuse material having a diamond shaped central opening. The diamond shaped central opening provides a pair of parallel current paths near-either side of the fuse element. The diamond shaped element has two oppositely disposed angles of approximately 37 2% degrees approximately eitherside edge of the fuse material.

7 Claims, 8 Drawing Figures CURRENT LIMITING FUSE WITH FUSE ELEMENT WITH A DIAMOND SHAPED CUTOUT BACKGROUND OF THE INVENTION This invention is related to electrical current limiting fuses in general and in particular to electrical current limiting fuses having central cutouts.

It is known in the prior art that current limiting fuses having areas of reduced cross section melt at the areas of reduced cross section when overload current has flowed in the fuse element for a predetermined period of time. After melting, an electric arc is established between nonmelted portions of the fuse for a period of time. This electric arc provides a back EMF or voltage drop in the electrical circuit which is effective for limiting the letthrough current after fusing which current is related to the inductance of the network being protected among other things. Examples of United States patents showing arrangements for current limiting fuses which are notched from the outer perimeter of the fuse element inwardly are U.S. Pat. Nos. 2,781,434 issued Feb. 12, 1957 to K. W. Swain and 3,601,739 issued Aug. 24, 1971 to Donald D. Blewitt who is also the inventor in the present case. An electrical fuse having central cutouts is disclosed in U.S. Pat. No. 3,671,909 issued June 20, 1972 by S. J. Kozacka et al. An example of a fuse element having both peripheral and central cutouts is disclosed in U.S. Pat. No. 2,720,567 issued Oct. 1 1, 1955 to L. Detch. All of the foregoing patents make general use of the areas of reduced cross section for fusing purposes but none teach optimization of the special relationship of the cutaway portion of the fuse element. U.S. Pat.'No. 2,868,040 issued Dec. 23, 1958 to W. S. Skeats teaches a fuse element having peripherally angularly notched fuse elements having inclined side walls which enclose an angle substantially within the range of 80 to 180. This patent teaches the optimization of the angle of the peripheral notch in order to extend the timebefore-fusing characteristic of the fuse during relatively light overload conditions while insuring a relatively high rate of rise of arc voltage during fusing. In U.S. Pat. No. 3,134,874 patented May 26, 1964 by F. L. Cameron peripheral V-shaped notches having an included angle of about 36 for a current limiting fuse are taught. The oppositely disposed V-shaped notches of the latter two patents teach a fuse element with a single narrow central core or isthmus between the V notches which links relatively heavier current carrying portions on eitherside thereof. This unitary area of reduced cross section is subject to radial twisting which may weaken or break the fuse element at its region of reduced cross section. U.S. Pat. No. 3,138,682 shows a diamond shaped central opening in a ribbon fuse where the oppositely disposed angles nearest the side edges of the fuse ribbon are obtuse providing for areas of reduced cross section which are long compared to the minimum transverse dimension of fuse material. This configuration is chosen to cooperte with a ceramic element which may be disposed on the fuse to provide certain performance characteristics for the fuse. The elongated regions of reduced cross sections provide spans of relatively weak material which are more likely to distort or break under the influence of twisting and vibration than shorter more compact spans. Japanese Patent SHO43 (1968) 12169 entitled TUBULAR FUSE WITH INNERTUBE in the name'of applicant Utsunomiya Electrical Factory Corporation discloses a fuse element having a central cutout therein but where the vertices of angles of the cutout near the edges of the fuse element are rounded with a relatively large radius of curvature compared with the length of aside of the angle. This gives elonaged areas of reduced cross section between larger adjacent areas in the fuse. It would be advantageous to provide a fuse element with a central cutout which provides two spans orregions of reduced cross sections between relatively larger sections of current conducting material in a fuse element to thereby increase the structural strength of the fuse element. It would also be advantageous to provide a fuse element with a central cutout or opening which is of a shape to optimize the trade-off between the pre-melting current carrying characteristic of the fuse element (to thereby maximize its effective rated current) and post-melting arc sus- SUMMARY OF THE INVENTION In accordance with this invention an electrical current limiting fuse is provided having one or more parallelfuse elements disposed therein between space terminals. A non-electrically conducting, tubular member is disposed between the space terminals to enclose the fuse elements. A pulverulent arc quenching material is added to enhance the current limiting action of the fuse. The fuse elements have spaced regions of reduced cross section which melt according to well-known fusing characteristics, e.g. the i k characteristic. The angles nearest the sides of the ribbon-like fuse material of the diamond shaped cutout are preferably 37% plus or minus 30 minutes, but may range between 30 and for satisfactory operation. The angle of 37 2 is chosen because it provides a trade off between the optimum pre-melting characteristic of the fuse and the optimum fusing characteristic of the fuse. In a fuse element the area or areas of reduced cross section are those areas which melt first during overload conditions because electrical current density in these areas is highest and melting temperature is attained sooner here than in any other portion of the fuse. This is because the areas of. reduced cross section represent the areas of highest resistance in the fuse element. If the areas of reduced cross section are elongated the net pre-fusing effect is to provide a very high resistance link between two areas of relatively low resistance. Generally speaking the longer the length of the area of reduced cross section the higher the electrical resistance of the entire fuse link. The higher the electrical resistance is the more heat is generated therein by any given amount of electrical current flowing therethrough. Since fusing or fuse blowing is a function of fuse element heating or temperature it can be seen that fuse elements with higher electrical resistance at the areas of reduced cross section are more likely to blow or melt before fuse elements having lower electrical resistance at the areas of reduced cross section. Said in 'another way this means that the fuses with less electrical resistance at the regions of reduced cross section have higher current ratings. If a diamond shape cutout is used in the fuse ribbon it is important for the vertex of the angles formed near the sides of the ribbon to be as sharp as possible. Said in another way it is important for the radius of curvature of the vertex to be very small compared with the length of the peripheral lines forming the angle. If the radius of curvature is relatively large, by the foregoing standard, the vertex itself provides a region of elongated resistance. Consequently in the present invention the lengths of the areas of reduced cross section are reduced insofar as they are effected by the radii of curvature of the vertices of the oppositely disposed angles of the diamond shaped cutout. It can also be seen that if the foregoing angles are made relatively small the amount of fusible material adjacent thereto increases and therefore the fuse resistance near the areas of reduced cross section decreases and the full load current rating of the fuse is thereby upgraded or raised. After fusing or melting at the area of reduced cross section, it is desirous to provide a sustained are between unmelted portions of the fuse element to generate a back EMF which acts to limit peak-Iet-through current during the fusing process. The lower the peaklet-through current the better because the let-throughcurrent if not retarded is available to cause damage in the network which is being protected bythe fuse. It has been found that a sustained arc of relatively high voltage is best produced by elongated areas of reduced cross section or said in another way the arc is more easily maintained for current limiting purposes if the angles of the diamond shaped cutout nearest the sides of the fuse element are made large. Consequently it can be seen that those physical characteristics of a fuse which provide optimum fuse operation before fusing tend to cause degraded fuse operation during the fusing or melting operation and vice versa. Consequently it is desirable to provide an effective trade-off between prefusing and fusing fuse characteristics. It has been found that a diamond shaped fuse element having sharp vertices at the angles nearest the outer edges of the fuse ribbon which angles are approximately 37 /2" provides an effective trade-off between fuse operating characteristics before fusing and during fusing. In addition it has been found that the mechanical and electrical properties of two parallel regions of electrically conducting material of reduced cross section between fuse sections of relatively larger cross section more than equals the electrical characteristics and mechanical characteristics of a fuse element of the same general size having opposed V-shaped notched portions taken out of the periphery thereof which provides only one region of electrically conducting material shape cutout which provides two parallel electrically conducting paths of a given cross sectional area has better electrical characteristics and mechanical characteristics than a fuse element with one path in the middle of the fuse element having a cross section area of twice the size. Two relatively short interlinking paths provide enhanced structural strength. The exposure of more surface area to are quenching material provides better are limiting characteristics and heat dissipation characteristics.

BRIEF DESCRIPTION OF THE DRAWING For a better understanding of the invention reference may be had to the preferred embodiments exemplary of the invention shown in the accompanying drawings in which:

FIG. 1 is a broken away side elevation of a fuse element having a diamond shaped opening in the central portion thereof;

FIG. la is an enlarged view of the vertex of the angle of FIG. 1;

FIG. lb shows another embodiment of a portion of the fuse element of FIG. 1 at the vertex 30;

FIG. 1c shows another embodiment of the fuse element;

FIG. 2 shows an elongated fuse element in side elevation;

FIG. 3 shows a side elevation partially broken away of a current limiting fuse structure;

FIG. 4 shows a cross section of the fuse structure shown in FIG. 3 along the line III-III and.

FIG. 5 shows the disposition of a fuse element between portions of oppositely disposed terminals such as shown in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings and FIG. 1 in particular there is shown a broken away portion of an electrical fuse element 10. Fuseelement 10 in this embodiment of the invention comprises a fuse element'for an electrical current limiting fuse. Fuse element 10 comprises electrically conductive material which is suitable for interrupting electrical current flowing therethrough when the current is at a predetermined value, as an example the fuse element 10 may be comprised of an alloy of silver or tin. Fuse element 10 comprises an upper edge or surface 14 and a lower edge or surface 16. In this embodiment of the invention fuse element 10 is shown as a relatively fiat ribbon-like strip where edges 14 and 16 represent the relatively thin elongated side surfaces of the ribbon. Fuse element 10 has disposed therein a central opening 18. In this embodiment of the invention central opening 18 comprising a parallelogram or diamond shape. The diamond sbaped opening 18 has four linear sides 20, 22, 24 and 26. Sides 20 and 26 are parallel and sides 22 and 24 are parallel. In addition in this embodiment of the invention sides 20, 22, 24 and 26 which together form the periphery of the central opening 18 are of equal length.

The peripheral sides 20 and 22 meet at a common point or apex 30 forming therebetween an angle. As will be described hereinafter with respect to FIG. la, the radius of curvature of the apex 30 is relatively small. Consequently the apex may generally be referred to as sharp. Apex 30 is spaced from surface 14. The fuse material between the edge or surface 14 and the apex 30 constitutes an electrically resistive region of reduced cross section compared with the size of the cross section of other portions of the fuse link as shown FIG. 1. Peripheral sides 24 and 26 of opening 18 are joined or meet at apex 32 forming an angle. It will be noted that apex 32 is spaced from edge or elongated surface 16 in a manner similar to the spacing of apex 30 and edge 14.

Electrical current may flow through fuse element 10 from right to left or left to right. In either case the electrical current must divide and flow generally in equal paths around the apexes 30 and 32 through the restricted regions between the edges 14 and 16 respectively.

Referring now to FIG. la an enlarged portion of the fuse element 10 in the vicinity of apex 30 of FIG. 1 is shown. Apex 30 comprises an arcuate portion 31 having a generally fixed radius of curvature r. Portions of the sidewalls 20 and 22 are shown intersecting the arcuate portion 3I tangentially. The angle theta (0) formed between the side portions 20 and 22 in the preferred embodiment of the invention is substantially within a range from 30 to 45. The preferred angle theta (6) is 37%? The tip of the apex 30 is spaced or separated from the outer surface or edge 14 of the fuse element by a distance d. Although the spatial relationships have been distorted for purposes of illustration it is to be understood that the radius of curvature r is to be significantly smaller than the distance d. In the preferred embodiment of the invention the radius of curvature r is one-fifth or percent of the distance d. Ideally apex should have a radius of curvature of zero. However it is recognized that fabricating techniques limit the actual size of the radius of curvature r to a practical lower limit. In the preferred embodiment of the invention the radius r is approximately 0.002 inches, the distance d is 0.0l inches plus or minus 0.0002 inches, the thickness of the fuse material (not depicted here) is approximately 0.02 inches, the distance from apex 30 to apex 32 is approximately 0.18 inches shown in FIG. I. It is desirous to make the radius of curvature r as small as possible so that the distance L depicted in FIG. la is minimized. The distance L measures an elongated path of relatively high electrial resistance for the pre-melting electrical current in the fuse element 10. The longer the path L is the higher the resistance of the fuse element, consequently the more heat which is generated during normal pre-fusing operating conditions. It would be very desirable to make L as small as possible because this would enhance the operation of the fuse during non-fusing conditions. This would have the advantage of increasing the current rating of the fuse element. However, it is desirous to have a fuse element which operates efficiently not only during pre-fusing operation but during the fusing operation as well. In the preferred embodiment of the invention fuse element 10 is utilized as an electrical current limiting fuse element which blows or fuses after an overload of electrical current and which generates an arc voltage thereafter which limits the further increase of electrical current which may occur due to electrical inductance in the various electrical circuits connected to the fuse element. The narrow region of the material between the arcuate portion 31 and the surface 14 of the fuse element 10 in Fig. represents the area of highest resistance in the fuse element and consequently is heated to a higher temperature by electrical current flowing therethrough and more rapidly than any other portion of the fuse element except another constricted region such as in the vicinity of apex 32 as shown in FIG. 1. Such being the case fuse element 10 blows, melts or fuses in the region or neck between the arcuate portion 31 and the surface l4. When this happens the electrical current is maintained through the hot plasma or are which exist between the remaining portions of the fuse element on adjacent sides of the recently melted region of reduced cross section. The arc generates a voltage which acts to retard or limit further current flow. The voltage is generally related to the length of the arc and generally increases therewith. It is desirous to provide a high voltage arc relatively quickly to thereby quickly begin to limit current which may be flowing in the plasma or arc and to also provide an arc voltage which is sustained for a relatively long period of time. Usually the arc will be completely extinguished and current will cease to flow before the current goes through a current zero. The diamond shaped opening characteristically shown in FIGS. 1 and la provides the arc voltage am plitude and time characteristics described. The are is prevented from rapidly expanding through the material around the region of arcuate portion 31 because in each succeeding increment of time the amount of material to be burned away or consumed by the arc increases. As can be seen this retards the relatively rapid propagation of the are into the regions on either side of the arcuate portion 31. This has the desirable characteristic effect of maintaining relatively high arc voltage for a sustained period of time. The are can be maintained for an acceptable period of time if the length K of the sidewalls 20 and 22 as shown in FIG. 1 is relatively long compared to the dimension d of the region of reduced cross section. This provides a greater amount of material to be consumed by the arc than if the dimension K was relatively smaller. In the preferred embodiment K is equal to or greater than l0 times d. It has also been found that the size of the angle theta (0) is preferably different for maximized prefusing operation of the fuse and maximize fusing operation of the fuse element. For example before fusing it is desirous to make the distance L as small as possible and to have as much fuse material available for current conduction as possible. This means that the angle theta (6) should be relatively small. However during a fusing operation it is desirous to increase the size of the angle theta (6) over the size that is best for pre-fusing operations. The reason forthis is as follows: if the angle theta (6) is made extremely large, approximately the portion of the fuse between arcuate portion 31 and surface 14 approaches the shape of a rectangular strip of material which provides a high are voltage upon blowing or fusing but which is extinguished almost immediately because of the limited volume of material to be consumed by the arc. On the other hand if the angle theta (0) is very small, approximately 0, the arc voltage is sustained for a relativelylong period of time because the amount of material to be consumed is very large. However the arc voltage is initially small because the arc cannot elongate at a rapid pace. Consequently it is desirous to have an angle of theta (0) during the fusing process which is sufficiently large to cause a relatively high are voltage in a relatively short period of time but which is not so large as to cause the arc voltage to collapse before the current to be limited thereby has completely been interrupted. The maximum angle for this purpose has been found to be significantly smaller than the previously described maximum angle for desirable steady state non-fusing operation. It is desirous therefore to provide a trade off between the angle theta (0) which is best for a non-fusing operation in the fuse element 10 and the angle theta (6) which is best for the fusing operation in the fuse element 10. It had been found experimentally that an angle theta in a range between 30 and 45 best accomplishes this purpose. It has further been found that an angle of 37% is the preferred angle for providing the trade-off previously discussed. It may be noted that the diamond shaped element of FIG. 1 resembles in some senses a fuse element section having 2 oppositely disposed V- shaped notches in the peripheral surfaces thereof.

which are spaced near the apexes thereof. It has been found in experimentations concerning the diamond shaped element that, all other factors being equal, an improvement is provided by the diamond shaped element if the following criteria are met:

(1) d K; and

The improvement takes two forms, first the diamond cutout element is stronger structurally than the V- notched element because there are two regions of support rather than one. This is true even though each region of support in the diamond shaped notched element is one-half or less the size of the single region in the V-shaped notched element. Also upon fusing rather than one relatively large are being generated carrying all the current during the fusingoperation two arcs of approximately the same voltage are formed having smaller cross sectional areas. It has been found that the cooling of the two arcs is easier than the cooling of the single arc. Since the voltage of the arc is related to how cool it is kept it can be seen that each of the plural arcs provides a higher current limiting voltage. Consequently the peak-let-through-current during fusing can be reduced for equal cross sectional 'areas of reduced cross section. The diamond shaped element may have the same resistance as the V-shaped notched element. However it has been found that the peak-let-throughcurrent for the V notched elements is to percent greater than that of the diamond shaped elements of the present invention for equal cross sectional areas and source voltage. Consequently a diamond shaped cutout in a fuse element can only be equaled in peaklet-through-current reduction characteristic by a V- notched element having 23 to 31 percent more cross sectional area. A reason for the improvement lies in the fact that the arcing path is separated into two parallel parts which enhance the ability of the surrounding material (which in the preferred embodiment of the invention may be a pulverulent arc quenching material such as sand) to absorb the energy and quench the arc.

FIG. 1C shows an embodiment of the invention where the side walls 20 and 22 undulate for better heat removal.

Referring now to FIG. 2 a whole fuse element 10 is shown. Fuse element 10 includes a number of diamond shaped cutouts 18 each of which are separated by a dis tance g. Fuse element 10 may also include a portion of tin or silver alloy material 34 which is provided to burn out or fuse first thus causing the necked or reduced portions of the fuse element to quickly burn out to meet thereafter. Fuse element 10 upon fusing will provide a series of two parallel arcs for each of the diamond shaped elements 18. The combined series are voltage acts to limit electrical current flowing through fuse element 10.

Referring now to FIG. 3 there is shown a cartridge type fuse 40 which employs the fuse element 10 as a constituent part thereof. Cartridge fuse 40 includes spaced electrical terminals 42 and 44 which have holes or openings 46 and 48 therein which are adapted for bolting the terminals to complementary electrical conductors or bus bars. An electrically insulating tubular casing 50 is provided for enclosing the fuse elements 10 and for spacing the terminals 42 and In a preferred embodiment of the invention the insulating casing 50 is cylindrical, tubular and the terminals 42 and 44 are snugly fit to either end thereof. In a preferred embodiment of the invention drilled holes such as 52 and 54 are provided in the outer casing 50 through which a pin or similar anchoring means may be inserted to engage similar holes 55 in the terminals 42 and 44 to lock the terminals to the insulating casing 50. The insulating casing 50 may be constructed of glass melamine or similar electrically insulating material. In terminal 42 is provided a fill hole 58 through which pulverulent arc quenching material 56 such as quartz sand may be provided to surround the fuse elements 10 so that upon fusing a fulgarite may be formed which absorbs heat during the fusing operation and thereafter acts as a dielectric insulator between the terminals 42 and 44. In the preferred embodiment of the invention the fuse 40 specifically satisfies the requirements imposed by NEMA FU 1-1966 for a class L fuse. In the preferred embodiment of the invention the fuse 40 is a 600 volt curent limiting fuse for conducting current between 601 and 6000 amperes and which has amaximum interruption rating of 200,000 amperes RMS symmetrical.

It will be noted in this fuse construction multiple fuse elements 10 are arranged in parallel between the tenninals 42 and 44. This provides more current carrying capacity during operating conditions and also provides a desirable fusing characteristic. It is generally known in the fuse art that the more parallel arclets that can be established between terminals the better the fusing action. Consequently multiple fuse elements 10 are provided in fuse 40. For the same reason the generation of 1 two multiple arcs in the fuse element, such as shown in,

FIG. 1, provides for a better fusing characteristic. This is related to the phenonomena of current commutation where a fuse element blows initially causing the other parallel fuse elements to be quickly overloaded and thus causing them to blow almost immediately thereafter. The first blown fuse element will conduct current and support an arc until the electrical resistance.

thereof is so high that the overload current seeks paths of lower resistance. This may cause the arc to be extinguished for a very short period of time in the recently blown fuse element while causing the non-blown low resistance fuse elements to fuse or blow thus generating their own arcs which in turn burn back causing the arc current to seek once again lower paths of resistance. Eventually the previously blown fuse element which is no longer conducting represents the lowest path of resistance and an arc is restruck thereacross. This effect is known as commutation or said in another way the jumping of an arc alternately between parallel fuse elements, during a fusing operation, as the resistance in each parallel changes in proportion to the amount of the fuseelement burned back by the preceding arc is commutation.

Referring now to FIG. 5, a portion of the fuse 40 is depicted as a fuse element 10 between portions of the two terminals or electrodes 42 and 44. Fuse element 10 is shown in a bowed configuration. The bowed configuration is utilized for many reasons, however, one of the most important reason is to allow for the contraction and expansion of the fuse element drawing normal nonfusing operating conditions when the electrical current flowing therethrough changes causing the heat generated therein to change correspondingly; It is well known that metallic fuse elements contract and expand in relationship to the heat generated therein.

Referring now to FIG. 4, a cross section of fuse element 40 cutaway along lines IVIV shown in FIG. 3 is depicted. The electrically insulating cylindrical tubular glass melamine outer shell 50 is shown in the cross section view. Behind it is shown a view of terminal 44 having fuse elements 10 which are radially arranged around the inside surface thereof. The .aligned holes or openings 54 and 55 are shown disposed radially around the outer perimeter of fuse terminal 44.

It is to be understood that fuse element may in some embodiments of the invention be adapted for use in fuses other than current limiting fuses. It is also to be understood that the fuse element 10 when used in a current limiting arrangement is useful over a wide range of fuse ratings in terms of power, voltage and current. It is also to be understood that the particular configuration of the terminals 44 and 42 as well as the structural characteristics of the tubular member 50 are not limiting nor is the chemical composition of the arc quenching material 56 limiting. lt-is also to be understood that the characteristic parallelogram or diamond shaped central opening shown in the drawings may in other embodiments of the invention be different. As an example the central opening may comprise only one angle and have anothercharacteristic shape in the periphery thereof for other electrical properties. It is also to be understood that the fuses of FIGS. 3, 4 and 5 are not limited to the fuse elements shown in FIGS. 1, 1a

and 2 but merely show a preferred embodiment for utilizing the fuse elements of FIGS. 1, la and 2.

The apparatus embodying the, teaching of this invention have several advantages. By utilizing theoptimized diamond shape central opening of the invention structural fuse strength is increased over peripherally V- notched fuses. Another advantage lies in the smaller distance g which can be maintained or provided between adjacent central opening because of the relatively small angle of the diamond shaped opening near the outer peripheries of the fuse element. This means that the merger of the arcs in both a parallel and series sense can be delayed. Another advantage lies in the general proposition that the more parallel electrical I arcs in a fuse or fuse element during fusing the better the operating characteristics of the fuse element during fusing. Another advantage is the ability of a fuse element having a diamond shaped central opening to maintain relatively high arc voltages for a relatively long period of time, without introducing extremely large initial arc voltages which might be destructive in themselves to the apparatus protected by the fuse element. Another advantage of the present invention is the improved cooling characteristics of the fuse element both during pre-fusing operation and during the fusing operation. Another advantage lies in the fact that the relative resistance-of the fuse element to electrical current flow during pre-fusing or non-fusing operation is relatively low compared with a fuse having an angular notched or diamond shaped portion therein which has a relatively large radius of curvature at the apex thereof because the smaller the apex the smaller the region of reduced cross section as measured in a longitudinal sense and consequently the smaller the linear or longitudinal resistance thereof. Another advantage lies in the fact that a fuse element having a diamond shaped central opening and having peripheral angles of relatively small radius of curvature can be stamped from fuse material having a thickness which is slightly larger than the distance between the. apex and one edge of the fuse element. Another advantage lies in the fact that the let-through-energy of a V-notched element ranges from 26 to 43 percent more than for an element with a diamond shaped central opening with all other factors being the same.

What we claim as our invention is:

1. An electrical fuse, comprising:

a. a tubular casing of electrical insulating material; I

b. spaced terminals disposed adjacent portions of said casing; and

c. a fuse element of electrically conductive material disposed within said casing and electrically connected to said spaced tenninals for interrupting electrical current flowing through said fuse when said current is of a predetermined value, said electrically conductive material having a central opening therein, said central opening enclosing as one part of the periphery thereof an angle substantially in the range of 30 to 45, said angle having an apex which is spaced from one surface of said electrically conductive material by a predetermined dis tance to thereby provide an electrically resistive region of reduced cross section, the bisector of said angle being perpendicular to said one surface within a predetermined range, the radius of curvature of said apex of said angle being smaller than said distance between said apex and said one surface by a predetermined amount to thereby reduce the electrical resistance of said conductive material adjacent said regionof reduced cross section, the length of at least one peripheral side of said angle being larger by a predetermined amount than said predetermined distance between said apex and said one side.

2. An electrical current limiting fuse, comprising:

a. a tubular casing of electrical insulating material;

b. spaced terminals disposed adjacent portions of said casing;

c. pulverulent arc quenching material disposed within said tubular casing; and

d. a fuse element of electrically conductive material disposed within said casing and electrically connected to said spaced terminals for interrupting electrical current flowing through said fuse when said current is of a predetermined value, said electrically conductive material having a central opening therein, said central opening enclosing as one part of the periphery thereof an angle substantially in the range of 30 to 45, said angle having an apex which is spaced from one surface of said electrically conductive material by a predetermined distance to thereby provide an electrically resistive region of reduced cross section which melts in the presence of said predetermined value of electrical current, the bisector of said angle being perpendicular to said one surface within a predetermined range, an electrical are being produced for limiting said current, said arc quenching material assisting in quenching said arc, the radius of curvature of said apex of said angle being smaller than said distance between said apex and said one surface by a predetermined amount to thereby reduce the electrical resistance of said conductive material adjacent said region of reduced cross section, the

length of at least one peripheral side of said angle v being larger by a predetermined amount than said predetermined distance between said apex and said one side.

3. The combination as claimed in claim 2 wherein said central opening is substantially in the shape of a parallelogram.

4. The combination as claimed in claim 2 wherein said angle is substantially 37.5.

surface within 4.

7. The combination as claimed in claim 2 wherein the length of said at least one peripheral side of said angle is equal to or larger than approximately 9.5 times the distance between said apex and said one surface. 

1. An electrical fuse, comprising: a. a tubular casing of electrical insulating material; b. spaced terminals disposed adjacent portions of said casing; and c. a fuse element of electrically conductive material disposed within said casing and electrically connected to said spaced terminals for interrupting electrical current flowing through said fuse when said current is of a predetermined value, said electrically conductive material having a central opening therein, said central opening enclosing as one part of the periphery thereof an angle substantially in the range of 30* to 45*, said angle having an apex which is spaced from one surface of said electrically conductive material by a predetermined distance to thereby provide an electrically resistive region of reduced cross section, the bisector of said angle being perpendicular to said one surface within a predetermined range, the radius of curvature of said apex of said angle being smaller than said distance between said apex and said one surface by a predetermined amount to thereby reduce the electrical resistance of said conductive material adjacent said region of reduced cross section, the length of at least one peripheral side of said angle being larger by a predetermined amount than said predetermined distance between said apex and said one side.
 2. An electrical current limiting fuse, comprising: a. a tubular casing of electrical insulating material; b. spaced terminals disposed adjacent portions of said casing; c. pulverulent arc quenching material disposed within said tubular casing; and d. a fuse element of electrically conductive material disposed within said casing and electrically connected to said spaced terminals for interrupting electrical current flowing through said fuse when said current is of a predetermined value, said electrically conductive material having a central opening therein, said central opening enclosing as one part of the periphery thereof an angle substantially in the range of 30* to 45*, said angle having an apex which is spaced from one surface of said electrically conductive material by a predetermined distance to thereby provide an electrically resistive region of reduced cross section which melts in the presence of said predetermined value of electrical current, the bisector of said angle being perpendicular to said one surface within a predetermined range, an electrical arc being produced for limiting said current, said arc quenching material assisting in quenching said arc, the radius of curvature of said apex of said angle being smaller than Said distance between said apex and said one surface by a predetermined amount to thereby reduce the electrical resistance of said conductive material adjacent said region of reduced cross section, the length of at least one peripheral side of said angle being larger by a predetermined amount than said predetermined distance between said apex and said one side.
 3. The combination as claimed in claim 2 wherein said central opening is substantially in the shape of a parallelogram.
 4. The combination as claimed in claim 2 wherein said angle is substantially 37.5*.
 5. The combination as claimed in claim 2 wherein said predetermined distance is equal to or larger than approximately 5 times the radius of curvature of said apex.
 6. The combination as claimed in claim 2 wherein said bisector of said angle is perpendicular to said one surface within 4*.
 7. The combination as claimed in claim 2 wherein the length of said at least one peripheral side of said angle is equal to or larger than approximately 9.5 times the distance between said apex and said one surface. 